I've been doing a lot of hinge repair on old leather books this week. The joint where the book opens is one of the weaker spots on most bindings and it's not a rare event to find boards broken or detached along the hinge area. A lot of fine 19th century bindings have particularly weak joints, as the leather was pared very thin for the sake of aesthetics. A quick, non-invasive repair for these bindings uses toned Japanese tissue to hinge the board back to the spine at the joint.
When I make a Japanese tissue match for leather, I mix acrylics with methyl cellulose and water. If I was making the same repair on a paper or cloth binding, I'd probably use acrylics with just water. You'll see images of the two different sets of books I've been working on this week.
So I diddle around and make color swatches with the Japanese tissue, dry them with a blow dryer and check the color against the leather. I should probably also point out here that before any diddling with the color begins, I usually consolidate the leather with a mix of Klu-cel G and SC-6000. This is important because both can darken the leather substantially. (Klu-cel G is a consolidant for dry leather and red rot. SC-6000 is a leather dressing, originally formulated as a shoe cream. S...C...get it?)
Once I get the right match, I brush out the color on the Japanese tissue with a piece of mylar or wax paper underneath. I usually let that air-dry, but there's no crime in getting the blow dryer out again. I cut the hinges to the width of the joint, and include a small flare at either end for where the tissue will wrap around the board edge.
I use straight PVA to glue out the hinge. I've also used Lascaux, another acrylic-based adhesive with slightly more reversible qualities. Wheat paste just won't work here. The hinge goes down, and is pressed lightly along the joint and around the turn-ins.